There is known a spindle headstock of a multiple-operation machine tool, comprising a housing which accommodates in a coaxial and rotatable arrangement, a spindle, a tie rod, and a bar. The spindle is hollow and kinematically coupled to a main drive. It accommodates a shank of a mandrel of a boring head. The shank is provided with an external tooth rim at one of its ends, which is intended to mesh with an internal tooth rim provided at the end of the tie rod. The tie rod is kinematically coupled to a drive by means of which it is axially located and the tooth rims of the shank of the boring head and of the tie rod mesh with each other. The tie rod can be turned for location relative to the spindle through an angle such that the teeth of the tooth rims are brought to a position one opposite another.
Inside the tie rod there is a bar kinematically coupled to a drive which moves it in the axial direction. At one end of the bar there is an internal tooth rim which serves to mesh with an external tooth rim at one end of a slide bar whose opposite end serves to convert the translation motion of the bar in the axial direction to radial motion of the carriage.
In order to bring the teeth of the tooth rims of the bar and slide bar in a position opposite one another, the bar is kinematically coupled to the tie rod so that the two can rotate simultaneously for location (cf. USSR Inventor's Certificate No. 933,290).
In this spindle headstock, the shank of the mandrel of the boring head is coupled to the tie rod by means of a spline joint. This joint cannot provide for an accurate location of the boring head, because it is impossible to eliminate clearances between the teeth. The resultant bending moment causes deformation of the mandrel of the boring head and of the spindle, which affects the machining accuracy. A removal or loosening of the shank may disturb the correct angular positioning of its teeth, which, in turn, may be the cause of an accident.
The manufacture of a tooth rim for the shank, which would always provide for an accurate angular orientation, involves a number of serious difficulties.